1. Field of the Invention
The invention relates to an apparatus for handling liquids and a process for operating the device.
2. Description of the Prior Art
An apparatus for handling liquids, in particular, can be an apparatus for proportioning and/or transporting and/or testing and/or processing liquids (e.g. chemically, physically or biologically). In the known apparatuses, operations are performed either manually or in a partially or completely automated manner. Thus, for example, manual pipettes, PC aided proportioning systems, and fully automatic proportioning stations are known for proportioning processes. Fully automatic handling stations (so-called xe2x80x9cworkstationsxe2x80x9d) are available to proportion, test, and process liquids.
Both in manual and automatical pipetting, the reception of liquid requires that the pipette tip be dipped into the liquid, on one hand, but only up to an immersion depth as small as possible, on the other, because the error in proportioning increases with the depth of immersion and the vessel receiving the liquid could be damaged if the pipette tip was dipped to a large depth. If manual pipettes are employed the user has to ensure this by accurately checking the depth of pipette tip immersion. Supervising equipment is used if automatic proportioning devices are employed.
Thus, it been known already to use conductive pipette tips or particularly conductive sensors the approaching of which to the liquid is monitored by a capacity measurement or the dipping of which into the liquid is monitored by a resistance measurement. These solutions, however, are bound to conductive liquids or require that the sensors be at small distances from the liquids or be dipped into them. Conductive pipette tips involve relatively high expenses for the consumable material. Inserting specific level sensors into vessels in which the liquid is may be annoying while the samples are processed.
From WO 0042384 A1, a detection system for liquid levels has been known for use in automatic workstations. The system has a light source and a photodetector which are directed to the liquid and towards each other, at an angle. The light source produces a light beam which, when reflected from the liquid surface, can be detected by the photodetector. The signal outputted by the photodetector varies depending on the intensity of the reflected radiation which is incident thereon. This output signal, in turn, will vary when the photodetector (and the light source) nears the liquid surface in that it increases initially and decreases afterwards because the reflected light beam travels from one side to the other side of the photodetector.
The beam path of the optical system is introduced through the opening of a vessel in which the liquid is. The required angle between the incident beam and the emergent beam limits its use to vessels which are of a relatively large diameter or a relatively small depth. In vessels having a relatively small diameter or a relatively large depth, however, the beam path would be interrupted at the border of the opening so that a measurement would no longer be possible.
Accordingly, it is the object of the invention to provide an apparatus for handling samples which makes it more convenient to determine the location of the liquid level in vessels of a relatively small diameter and/or a large depth. It is a further object of the invention to provide a process for operating an inventive apparatus.
The object of the invention is achieved by an apparatus for handling liquids that has
a lighting installation for illuminating an approximately punctiform illumination point in the room,
an approximately punctiform light-receiving device having a photodetector for providing a measuring signal dependent on the intensity of the light received,
an imaging system for imaging the illumination point onto the approximately punctiform light-receiving device, and
an evaluation device for detecting the approach of an interface between two media of different refractive indices to the illumination point by evaluating the measuring signals provided by the photodetector.
In the inventive device, the approximately punctiform illumination point is imaged onto the approximately punctiform light-receiving device. As a result, the intensity of the light radiation received by the light-receiving device will vary and, hence, so will the measuring signal provided by the photodetector if an interface enters the illumination point between two media of different refractive indices. Thus, it can be ascertained whether or not there is an interface in the illumination point. This allows to establish the position of an interface which can be a liquid level, i.e. the interface between a liquid and air, for example. It further allows to establish the location of a surface of an object which can be non-covered, for example, or be covered by a medium transparent to the light of the lighting installation (e.g. a vessel bottom covered with a liquid).
To determine the position of an interface, it is possible to vary the relative position of the illumination point and the interface until the interface is in the illumination point. It further is possible to scan the surface of an object using the illumination point in order to judge on the position of the entire object and/or its identity on the basis of individual values or the course of the measuring signal.
Thus, the inventive apparatus specifically permits to determine the height of the liquid level in vessels (e.g. reaction vessels and in indentations of microtitration plates), the position and identity of vessels (e.g. reaction vessels and microtitration plates), and the position and identity of tools and aids (e.g. pipette tips in a rack). Since the light beam of the lighting installation and the imaging system are directed coaxially to the illumination point a non-contacting detection of interfaces is possible from a major distance and at a low lateral space requirement. This makes it easier to detect the liquid level in vessels having a relatively small opening and/or a relatively large depth.
To detect a liquid level free from trouble caused by adjacent vessel walls, it is possible to appropriately use light of a single wavelength to which a liquid (e.g. water) is opaque.
The interfaces detectable are both diffusely reflecting interfaces (e.g. dull surfaces) and substantially directionally reflecting interfaces (e.g. glossy surfaces or liquid surfaces). The measuring signal is particularly strong, above all, for substantially directionally reflecting interfaces if, according to an aspect, the surface normal line at the place where the interface is scanned by the illumination point is approximately coaxial to the illuminating light beam and the optical axis of the imaging system.
Preferably, the lighting installation comprises a light source of its own which can be a laser, LED or small bulb, for example.
The illumination point may be lighted by a linear light beam which can be produced by means of a laser, for example. On the linear light beam, the imaging system will then define a punctiform illumination point, which is imaged onto the approximately punctiform light-receiving device.
According to an aspect, the lighting installation also comprises an imaging system which images the light of a punctiform light source onto the illumination point. As a result, the illuminating light beam (or xe2x80x9clight bundlexe2x80x9d) has its largest intensity in the illumination point. Along with imaging the illumination point onto the light-receiving device, this results in a measuring signal which is particularly strong.
According to an aspect, the light of the punctiform light source is fed to the imaging system via a beam splitter and the same imaging system images the illumination point onto the light-receiving device via the beam splitter. This realizes an incident-light measurement. Since there is only one imaging system the expenditure involved is relatively low.
According to an aspect, the lighting installation has a diaphragm stop and/or an optical waveguide in the optical path of the light source, the output of which is formed by the punctiform light source.
According to an aspect, the light beam lighting the illumination point has an aperture angle of 8xc2x0 or less so that the light beam is adapted to be introduced into vessels of a relatively small opening diameter or a large depth without undergoing a fade-out in the external area.
According to an aspect, the distance of the illumination point from the imaging system is 100 mm or more, which enables a non-contacting measurement to be made for the liquid level in many ordinary vessels.
As a principle, the punctiform light-receiving device may be a photodetector having a particularly small light-sensitive area. According to an aspect, the punctiform light-receiving device has a diaphragm stop which determines the size of the receiving area.
According to an aspect, the evaluation device has means for filtering the measuring signal provided by the photo-detector. This helps reduce a noise of the measuring signal and an impact of extraneous light.
According to an aspect, the apparatus is an automatic apparatus (e.g. a proportioning station or workstation) for handling liquids. Such an automatic station allows to control courses therein, e.g. the dipping of pipette tips into vessels, the processing of liquids in certain vessels, etc. because it detects the location and/or identity of liquids and/or objects.
According to an aspect, the apparatus has a shifting device to displace the relative position of the illumination point and the interface towards the optical axis of the imaging system and/or in a cross direction thereto.
The displacement of the relative position of the illumination point and the interface may be caused by the shifting device in various manners. According to an aspect, the relative position of the entire optical system formed by the lighting installation, the imaging system, and the light-receiving device and the interface is adapted to be displaced by means of the shifting device. This may be utilized for both a displacement towards the optical axis of the imaging system and a displacement transverse thereto. To this end, the entire optical system and/or the interface may be shifted to another place, e.g. by means of a specimen slide. According to an aspect, the shifting device has a zoom objective disposed in the imaging system for a displacement towards the optical axis. For a displacement transverse to the optical axis, the shifting device may have at least one scanning mirror in the imaging system.
According to another aspect, the shifting device is driven by a motor, e.g. for an integration into an automatic apparatus for handling liquids.
According to another aspect, the evaluation device controls the displacement of the relative position of the illumination point and interface by the shifting device. Then, displacement may be performed depending on the measuring signals, e.g. to adjust the illumination point to the interface and/or move it along the interface.
According to the inventive method for operating an inventive device,
the distance between the illumination point and an interface is varied, the maximum of the measuring signal is determined while varying the distance, and the location of the illumination point is determined as the location of the liquid level at the maximum of the measuring signal, and/or
the illumination point is displaced substantially in parallel with the interface, individual values or the course of the measuring signal are determined while the displacement is made, and the position and/or identity of the interface are determined while referring to the values or course of the measuring signal.
This method allows to determine the location of a liquid level and/or fixed object and/or the identity of a fixed object. An identification of the interface also identifies the liquid or object.
According to an aspect, the position and/or identity of the interface are determined by a comparison of the values measured to reference data on the configuration and/or the reflective characteristics of the interface. The reference data may be stored, for example.
Finally, according to an aspect, the illumination point is adjusted to a reference surface and the reference signal is measured that serves as a reference point for the measuring signal.